This invention relates to the barometric altimeter which measures accurate pressure altitude according to the standard atmosphere by electronic means. A basic version of my measurement technique is disclosed in my first invention: Japanese Pat. No. 785,728 "Barometric Altimeter" and U.S. Pat. No. 3,693,405 "Barometric Altimeter", in which the measured altitude is represented by the time interval of two events at which an exponentially changing electrical signal becomes equal to two output signals from a pressure sensor, one signal produced at the reference altitude and the other signal produced at a point at which the altitude is to be measured. In this measurement, a correction must be made on the measured value according to the average temperature of the atmosphere between the reference altitude and the point of measurement.
My second invention: U.S. Pat. No. 3,958,459 "Barometric Altimeter" discloses an improved method of altitude measurement, in which the correction for the average temperature is not needed, based on the standard atmosphere.
In the present invention, a pressure transducer which has a linear or non-linear characteristics expressed by: EQU S=kP.sup.a +b (1)
where
S: output of the pressure transducer PA1 P: pressure, PA1 a, b, k: constants
is incorporated in a pressure sensor which also contains a output stage by which the offset b of equation (1) is compensated for to produce an output signal: EQU Vs=kP.sup.a ( 2)
where Vs: output of pressure sensor, offset compensated.
This is the same as the pressure sensor used in my second earlier invention. The second invention uses a quasi-exponential function signal, generated by a quasi-exponential function generator (acronymed as QEFG hereafter) in place of the exponentially changing signal of my first earlier invention and utilizes a pressure sensor having a characteristics expressed by equation (2). The exponentially changing signal of my first earlier invention has a time constant of a fixed value, whereas the quasi-exponential function signal of my second earlier invention has a time constant which changes with time.
FIG. 1 shows the principle of altitude measurement of my second invention, in which the horizontal axis shows the time t, the vertical axis represents voltage, and v is the output of the QEFG, V.sub.s0, V.sub.s1 are respectively the outputs of the pressure sensor at the reference altitude and at the altitude to be measured. Times t.sub.0 and t.sub.1 show respectively the times at which v becomes equal to V.sub.s0 and V.sub.s1. Then the time interval t.sub.1 -t.sub.o represents the measured altitude.
FIG. 2A shows the basic measuring circuit of my second invention. In FIG. 2A, QEFG generates the quasi-exponential function signal v, V.sub.s0 is the signal which would be produced by the pressure sensor at the reference atitude, S is the pressure sensor and produces signal V.sub.s1 at the altitude of measurement. The first comparator Comp. 0 generates a first coincidence signal at the time t.sub.0 when v becomes equal with V.sub.s0 and controls the counter Count. to start counting and the second comparator Comp. 1 generates a second coincidence signal at the time t.sub.1 when v becomes equal with V.sub.s1 and controls the counter to stop counting. During the time interval between t.sub.0 and t.sub.1, the counter Count. counts the pulses generated by the oscillator Osc. By properly selecting the frequency of the pulses generated by the oscillator Osc., the number counted by the counter Count. corresponds to the measured altitude.
FIG. 3 shows the principle of QEFG. In FIG. 3 the charging switch S.sub.c is turned on and discharging switch S.sub.d is turned off before discharge begins (that is the time t&lt;0 in FIG. 1), and the capacitor C is charged by the voltage V. At the time t=0, the switch S.sub.c is turned off and the switch S.sub.d is turned on and the electric charge in the capacitor C begins to discharge through the discharge resistor circuit R. By controlling the discharge resistor to decrease its resistance R linearly with time until the time which corresponds to the altitude of 11,000 m and thereafter to keep the resistance of the discharge resistor R constant up to the time which corresponds to the altitude 20,000 m, an altitude measurement up to 20,000 m is made by the QEFG of FIG. 3.
FIG. 4 shows one example of a QEFG, the principle of which is shown by FIG. 3. The variable resistance R of the discharge resistor in FIG. 3 is replaced by a circuit containing resistors r.sub.0, r.sub.1, r.sub.2, r.sub.3, . . . r.sub.n-1 of same resistance, and a resistor r, and switches S.sub.d0, S.sub.d1, S.sub.d2, . . . , S.sub.dn. By means of turning on the switches S.sub.d0, S.sub.d1, S.sub.d2, . . . , S.sub.dn in sequence with a equal time interval, a voltage v' which is close to the quasi-exponential function voltage v is generated at the terminals of the capacitor C.
FIG. 5 shows the relationship between of v and v'. As shown in FIG. 5, by setting the initial value V' of the approximate quasi-exponential voltage v' a little larger than the initial value V of quasi-exponential voltage v, the overall approximation is improved. The error due to the use of approximate quasi-exponential voltage v' instead of v depends on the number of sections n into which the resistor R of FIG. 3 is divided as shown by r.sub.0, r.sub.1, r.sub.2, r.sub.3, . . . , r.sub.n-1 of FIG. 4. When the number of sections n is selected to be 12, for measurements of up to 36,000 ft, the largest calculated error of the system is 6 ft at lower altitudes and 8 ft at higher altitudes.
FIG. 6 shows another example of a QEFG, the principle of which is also shown by FIG. 3. The QEFG of FIG. 6 is a hybrid system using analog and digital technology. In FIG. 6, the capacitor C which has been charged with voltage V begins to discharge at the time t=0 through a discharge resistor circuit composed of a resistor R" and a switch S.sub.w, where the switch S.sub.w is controlled on and off repeatedly by control signals from a controller Cont. so that the effective resistance of the discharge resistor circuit decreases linearly with time, and a voltage v very close to the ideal quasi-exponential voltage is generated at the terminals of the capacitor C.
FIG. 7 shows one example of the controller Cont. of FIG. 6. In FIG. 7, R.sub.0, R.sub.1, R.sub.2, R.sub.3 are registers and D is a detector circuit which detects the change of the content of the register R.sub.0 at each cycle of operation of the controller. Predetermined initial values are stored in the registers R.sub.0, R.sub.1, R.sub.2 and R.sub.3, and one cycle of the operation of the controller is performed by adding the content of register R.sub.3 to R.sub.0, adding the content of R.sub.2 to R.sub.1 and by adding the content of register R.sub.1 to R.sub.0 a number of times according to the value detected by the detector circuit D. To implement this controller, it is possible to design so that the change of the content of the register R.sub.0 at one cycle always remains below 1. Under such design, it is sufficient for the operation that the detector circuit D detects only the change of the content of the order of 2.sup.0 of the register R.sub.0, and when such a change is detected, the detector circuit D controls the addition of the content of the register R.sub.1 to the register R.sub.0. By controlling the switch S.sub.w of FIG. 6 to turn on only when the above-noted change is detected by the detector circuit D, the effective resistance of the discharge resistor circuit, composed of registor R" and switch S.sub.w, decreases linearly with time, and the quasi-exponential function voltage v is generated at the terminals of the capacitor C.
Although most of the pressure transducers proposed for altimetry show nearly linear characteristics, they are found to have some degree of non-linearity when examined in detail. Such curvature of the characteristic curve cannot be ignored for a altitude measurement of high accuracy. The characteristic equation (1) or (2) is especially suitable to represent such slightly curved characteristics with high accuracy. As shown by FIG. 8(A), when the characteristics of pressure P VS sensor output S is strictly linear, it is represented by a=1 in the equation (2), when the characteristics is curved upward, it is represented by a&gt;1, and when curved downward, it is represented by a&lt;1 in the equation (2).
Referring to FIG. 3, in the time region of t&gt;0, the resistance of the discharge resistor circuit R is changing. Let's denote by R the instantaneous value of the changing resistance, then the instantaneous value of the time constant of the discharging circuit of FIG. 3 is CR. If this QEFG has been designed for use with a perfectly linear pressure sensor, where a=1, then the altitude measurement with a nonlinear pressure sensor, where a.noteq.1, is made with a correct theoretical accuracy by means of changing the instantaneous time constant to CR/a. This is disclosed in my second invention. Changing the instantaneous time constant is done either by changing the capacity C to C/a, or by changing instantaneous resistance R to R/a, or by changing the combined value CR to CR/a. In the foremost case, the QEFG will be as shown by FIG. 9, which shows that, if the value of "a" of equation (2) changes, the capacity of QEFG would change so as to be inversely proportional to a.
The characteristics of the pressure transducer changes with temperature variations. For example, suppose a characteristic curve of a pressure transducer at a temperature is represented by the solid line curve of FIG. 8B, then, that characteristic curve at a different temperature will be as shown by the dotted line curve. It has been extremely difficult to measure altitude with high accuracy by those pressure transducers on an aircraft where the temperature changes adversely, without using a thermostat. To cope with this difficulty, there have been pressure transducers with thermostats. But they have disadvantages of using additional power for heating, and additional time for heat up. In addition to the pressure transducer, the elements of the QEFG also change with temperature variations. A temperature stabilized QEFG needs special components and results in a high cost. A thermostat equipped QEFG uses additional power and has an inherent time lag. Improvements to cope with these problems have been awaited.